Explanation:

The correct answer is option 'B' - False.

Explanation in detail:
The theory of friction explains the resistance to motion between two surfaces in contact. It states that when two surfaces come into contact, there is a force that opposes their relative motion. This force is known as friction.

When studying the theory of friction, it is common to use a block to represent one of the surfaces. However, in the explanation of the theory of friction, the assumption is made that the upper portion of the block is considered to be rigid. This means that the upper part of the block does not deform or change shape when a force is applied to it.

Why is this assumption false?
In reality, no material is completely rigid. All materials have some degree of flexibility and deformability. When a force is applied to a material, it will experience some amount of deformation. This deformation can affect the frictional behavior between the surfaces in contact.

In the case of the block used in the explanation of the theory of friction, if the upper portion of the block is not allowed to deform, it does not accurately represent the real-world scenario. The block would not exhibit the same frictional behavior as a real surface would.

Importance of considering deformability:
Considering the deformability of materials is important when studying friction because it affects the contact area and the distribution of forces between the surfaces. When two surfaces are in contact, the actual contact area may be smaller than the apparent contact area due to the deformations that occur. This can significantly affect the frictional forces between the surfaces.

Therefore, it is important to consider the deformability of materials when studying friction to obtain accurate and realistic results. Ignoring the deformability of the upper portion of the block in the theory of friction would lead to incorrect assumptions and predictions about the frictional behavior between surfaces.

Conclusion:
The assumption that the upper portion of the block used in the explanation of the theory of friction is rigid is false. In reality, materials are not completely rigid, and considering their deformability is crucial for accurately understanding and predicting frictional behavior.